Energy absorption materials are commonly encountered in everyday life: from bubble wrapping and Styrofoam, classically used for packaging protection; to car bumpers, fenders and headrest foam used in vehicle industry; to shoe soles, human protection gear and sports equipment. The use and need for energy absorption materials are countless as are the different materials, and at times, mechanisms used for this purpose. Generally, energy-absorbing pads are made from polymeric foams, or sometimes, polymeric materials (rubber). At times they also include mechanisms like fluid pockets, piston-like fixtures, springs, and any combinations of the above.
The reason for the many varieties is that each pad or material is tailored for a specific type of impact. The difficulty of choosing the right material arises from the fact that compliant materials, which are more comfortable to wear, are very good at absorbing small impact energies, but “bottom out” or saturate and became no longer useful if input energies are large. On the other hand stiffer materials are able to absorb a large amount of energy but are typically uncomfortable as they do not readily conform to shape, and therefore do not adhere to the surface. This can be a big problem for human protective gear like orthopedic supports, bulletproof vests, helmets, etc. This is a well-known problem commonly referred to as the “conflict of stiffness”.
Energy absorption materials can be classified as “passive” and “active” materials. Passive materials have specific material characteristics that cannot be varied; for example foams or polymeric materials. These are the most common class of energy absorbing materials and are also the simplest. Active materials on the other hand can be controlled and their material properties changed in order to tailor their energy absorbing characteristics to different impact loading. A few examples of these materials are described in the U.S. patents discussed below. These material can be optimized, a priori, to work over a range of impact energies, and are normally characterized by only two states, an “off state,” compliant and comfortable; and “on state,” stiff and energy absorbent. The change in the amount of energy absorption or the “gain” is chosen a priori. The mechanism of activation is normally the impact itself, rather then external activation and requires specific and ideal conditions such as large deformations. These materials are efficient, but they rely on passive activation and cannot be “actively tuned” through external user control to best match a given operational setting.
In the myriad applications of energy management there is the need for a controllable, adaptive and active energy absorbing material. For example, such a material with these characteristics could be used in shoes, for which the stiffness of the sole could be actively adapted to the user's weight and the type of terrain encountered. Another possible application could be in sports equipment; for example, hollow ski bodies filled with this material would allow the skier to actively control the ski's flexibility. Another class of potential applications for these materials is in medical devices wherein, if the material could be tuned to be extremely stiff, it could be used to create adjustable splints or braces. Imagine putting on a soft sleeve to the patient's arm or leg until it is in the right position and then turning the material on and it becomes a rigid cast. Another possible application could be in the field of firearms; for example, these active tunable materials could be used to absorb the shock from a gun's recoil.
Numerous earlier patents have dealt with the development of materials for “passive” energy management; for example, absorbing, dissipating and/or shunting energy. Fluids and fluid flow have been an integral part of many of these patents because of their energy dissipation and load shunting characteristics. Some examples are U.S. Pat. No. 5,564,535 issued to J. N. Kanianthra, U.S. Pat. No. 3,672,657 issued to B. O. Young et al., and U.S. Pat. No. 5,915,819 issued to E. Gooding that describe structures comprising of a plurality of fluid-filled cells or reservoirs, wherein energy dissipation is achieved through restriction of fluid-flow through orifices or in-between cells and reservoirs. Fluids have also been used to form the underlying matrix of the structure itself so as to provide a damping or a load shunting effect. For example, World Patent No. 09949236 describes an energy absorbing material wherein elastomeric capsules are dispersed in a matrix liquid.
However, none of these structures provide energy management over a large dynamic operating range. As the magnitude of applied forces increases, an increasing device stiffness or thickness is required to prevent the material from saturating or “bottoming out”. These structures thus necessitate a trade-off between user comfort and device rigidity, and are prone to changes in ideal external conditions. On the other hand, devices employing a shear-thickening (dilatant) fluid can be designed to be comfortable and compliant under lower applied stresses and naturally increase in rigidity as the applied loads or forces increase. As described below, the present invention employs a shear-thickening fluid which has a self-adjusting viscosity and hence its utility extends to a large dynamic operating range.
U.S. Pat. No. 5,545,128 issued to W. C. Hayes et al. and U.S. Pat. No. 6,701,529 issued to L. J. Rhoades et al. also employ shear-thickening (dilatant) fluids for energy dissipation by incorporating them in bladders, envelopes or cells. These structures are able to provide a large operating range; however, they suffer from a lack of controllability tunability and adaptability. They are “passive,” that is are dependent upon and responsive to ideal and specific external loads only. Thus, under a given set of external conditions and forces, the energy dissipation in these devices is passively fixed and independent of the user demand or specified requirements. Further these materials cannot be used in medical devices such as splints and braces since there is no mechanism to activate them into a rigid cast.
U.S. Pat. No. 4,759,428 issued to K. Seshimo, U.S. Pat. No. 4,852,533 issued to F. Doncker et al., and U.S. Pat. No. 5,645,138 issued to H. Takima et al. describe systems in which dilatant materials are used to suppress vibrations (and respond differently to different frequency vibrations), but provide no mechanism for controlling stiffness in order to manage anticipated impacts. In these prior systems, energy absorption is optimum only at a single deformation rate rather than for the different rates that practical devices will encounter during use.